Process for preparation of silver alloy catalyst



PROCESS FOR PREPARATION OF SIL ALLOY CATALYST Wendal A. Alexander, Ottawa, Ontario, Canada, assignor to National Research Council, Ottawa, Ontario, Canada, a body corporate of Canada No Drawing. Application July 20, 1954 Serial No. 444,670 1 Claims. (Cl. 25 2475) from, or may be learned by practice with the invention,

the same being realized and attained by means of the steps, processes and compositions pointed out in the appended claims.

The invention consists in the novel steps, processes,

compositions and improvements herein shown and described.

Heretofore it has been proposed to make catalysts for the oxidation of ethylene by granulating a silver-calcium alloy and thereafter etching away at least some of thecalcium and further treating the remainder of the alloy so that it is rendered catalytically active. Sheets of catalytic material of the silver-calcium alloy have also been formed by adhering a granular catalytic material to a heat conductive metal base, such as analuminum or silver sheet, but such catalysts are prepared with some difliculty and frequently failin service due to thetendency of the granular material to blister and free itself from the supporting metal sheet after being heated for long periods of time. tively soft and present some difficulty in being formed into granular particles.

It has also been proposed to form the catalysts by dipping sheets of metal into a bath of molten metal, e. g. by dipping a silver sheet into a bath of molten calciumrnagnesium alloy to form a silver-calcium-magnesiumalloy on the faces of the silver sheet, or by dipping an aluminum sheet into a molten bath of silver-calciummagnesium alloy, either method providing a silver-alkaline earth metal alloy which may be rendered catalyti cally active. However, with these methods it is difficult to control the thickness of the alloy on the metal base so as to produce a uniform catalyst layer on the-base metal. Furthermore, the annealing procedure with such dip-coated catalysts often gives rise to problems caused by the flowing of the surface alloy layer.

The various disadvantages of the prior art are substantially avoided by the process of the present invention which provides a uniform layer of calcium or other alka line earth metal on the silver sheet or particles serving as a base, insures complete adhesion of the catalyst layer to the silver surface of the base, and simplifies the annealing so that a uniform catalystis readily produced. A

The silver-calcium alloys also are rela-" I United States Patent further object of the invention is the provision of a novel I and improved process of forming a catalyst body which is especially useful in the oxidation of ethylene and other ethylenic hydrocarbons. The invention further provides a process of forming a silver base catalyst in which the catalytic alloy layer may be accurately controlled.

as to thickness so as to insure uniformity of the, catalyst body.

According to the, present invention, silver, or a metal 1 2 having a silver surface is coated under relatively high vacuum conditions preferably in the order of 500 microns (mercury) or less by a vaporized alkaline earth metal, such as calcium, and the process being continued for'a sufiiciently long time to provide the desired thickness of alkaline earth metal condensed upon the surface of the silver sheet or granular particles. The thickness of the condensed alkaline earth metal may be accurately controlled by controlling the time and intensity of the heat supplied to the alkaline earth metal or by introducing'an inert gas into the vacuum chamber when the desired thickness of deposit has been reached. In accordance with my invention, the alkaline earth metal condensed upon the silver sheet or particles and the silver are heated under suitable temperature condition causing the forma-" tion of a silver-alkaline earthmetal alloy.

Various methods may be used for alloying the silver and alkaline earth metal. As will be well understood by those in the field, the necessary alloying temperature de*-- pends upon the particular alkaline earth metal that is to be alloyed with silver. For example for calcium a suit-' able alloying temperature in the range 450 to 640 C. may be used.

In accordance with one method the silver is heated to an alloying temperature during the vacuum deposition of the alkaline earth metal thereon causing the deposited alkaline earth metal to alloy with the silver.

Alternatively, the alkaline earth metal may be deposited on cold or unheated silver, after which, the alkaline earth metal coated silver is heated to an alloying temperature in an inert atmosphere (argon, helium, ammonia, carbon tetrachloride, carbon dioxide, etc.) to complete the alloying of the alkaline earth metal with the silver.

A third modification is to heat the silver below the alloying temperature during the vacuum deposition of the alkaline earth metal on the silver, and subsequently, heat-i ing the alkaline earth metal coated silver to an alloying temperature to complete the alloying of the alkaline earth metal with the silver.

If the alloy is in sheet form, it is preferably cut to the desired size for use, while if in granular or particulate form, the alloy need only be activated for use.

The catalytic alloy is thus prepared for activation and may be activated by etching away the calcium orv other alkaline earth metal and such other treatment as:

desired.

It will be understood that the foregoing general descrip I tion and the following detailed description as well are exemplary and explanatory of the invention but are not restrictive thereof.

Referring now in detail to the present preferred and illustrative manner of carrying out the process of the present invention, the process will first be described with reference to the preparation of a catalyst in sheet form suitable for use as fins on reactor tubes for the oxidation r of ethylene in accordance, with the process described in the prior patents to Cambron and McKim No. 2,562,857 and No. 2,562,858. I

metal evaporator unit, and is substantially uniformly spaced from the metal to be evaporated. The metal to.

be evaporated is preferably relatively pure calcium, al-

line earth metals. Suflicient metal is provided to produce a layer about 0.004 to about 0.008" inthickness on the surface of thesilver sheet, although these limits Patented July 8,

for the thickness of the alkaline earthmetal are not at all critical. Thus from 3 to 30 grams of calcium may conveninently be provided for each 100 sq. in. of surface of silver sheet to be coated with calcium by evaporation.

The evaporation unit is then evacuated until the pressure has been reduced to the order of 500 micron or less, and preferably about 2x10 mm. mercury pressure.

The silver sheet is preferably heated to insure a more uniform coating of the surface of the silver sheet with the calcium, and is preferably heated to a temperature at which a silver-calcium alloy is formed, as for example 585 C.

The calcium or other alkaline earth metal is preferably provided with a large surface to facilitate evaporation. Generally, it is in form of a bar and is heated to vaporize it and cause the vaporized calcium to condense on the freshly cleaned surface of the silver sheet. Use of a molten pool of calcium with a temperature difference between silver and calcium is also effective for depositing the calcium on the silver.

After the formation of the silver-calium alloy sheet, it is allowed to cool, after which it is removed from the vacuum chamber of the evaporation unit and may be handled in air without undue oxidation. The coated sheet is then cut into parts of the desired shape to form the eventual fins or other catalytic members. Then the surfaceof the calcium coated silver is roughened and extended as by rolling with a coarse knurling tool.

Thereafter from about 5% to substantially all of the calcium metal is removed from the alloy formed by the silver and calcium, and this removal of the calcium may be effected successfully by steam, water or acid treatment or by other procedures as set forth in the prior patent to Cambron and McKim No. 2,562,858.

The catalyst elements may be treated with a mixture of 90% nitrogen and steam at a temperature of 350 C. for one hour and then with steam only, for three hours. The oxidized alloy may then be treated with aqueous acetic acid 20% by volume or other acid for one and one-half hours and the solution decanted. The resulting catalytically activated alloy is washed in distilled water and dried.

The steam or hot water treatment of the alloy surface followed by the leaching or etching treatment with acid causes the surface of the silver-enriched surface to become porous, or sponge-like due to the preferential removal of a substantial portion, at least 5%, and oftentimes all but a trace, of the alkaline earth metal leaving the silver substantially unaffected, while the subsurface portions of the alloy will, at least prior to use, contain a greater proportion of alkaline earth metal.

Instead of using flat, fine silver sheet, the catalytic elements may be vapor-phase coated with calcium metal while the catalytic elements are in their final shape, and may be in the form of hemispheres or cup-like objects, or formed as cylinders with smooth or corrugated surfaces, depending upon the eventual shape desired for the catalytic elements.

Instead of using sheets of fine silver, sheets of aluminum, stainless steel or other suitable metal may be used coated with a relatively thick layer of fine silver. Thus the base metal may be a sheet of aluminum or stainless steel having a thickness of about ten thousandths of an inch, and coated on both sides with a welded, electrodeposited or sprayed layer of silver about .005" or more in thickness, thereby providing a silver clad base metal which has a surface coating of substantially pure silver,

although the thickness here given is illustrative only. In

general, the thickness of the silver layer is at least equal to and is preferably thicker than the film of alkaline earth metal to be condensed thereon so as to insure sufiicient' silver to form a silver-rich alloy after annealing.

Other alkaline earth metals than calcium may be similarly employed, including mixtures .of alkaline earth metals. Thus, the metal to be evaporated onto the silver may be a mixture of calcium and magnesium containing 78.5% calcium. A suitable alloying temperature for this mixture with the silver is 5 85 C.

In case magnesium is to be evaporated to form a vapor phase coating on the surface of the silver, the silver is preferably heated to a temperature not exceeding about 745 C.

In case the catalyst is to be eventually used as a fluidized bed, or in a packed reaction chamber, the silver is preferably initially formed into the desired size of particles which may conveniently range from about 0.4 to about 1.7 mm. in size, preferably 0.10 to 0.20 mm., and are preferably formed of substantially pure silver. These silver granular particles are then vapor-phase coated with calcium, one of the other alkaline earth metals, or with a mixture of the alkaline earth metals, by evaporation of the alkaline earth metal onto the surface of the silver granules, said silver particles being heated to a sufficient alloying temperature so as to insure formation of the alloy. The silver alloy particles are then subjected to the treatment with water, steam or acid so as to activate the catalytic alloy and remove from about 5% to substantially all of the alkaline earth metal thereby rendering the particles catalytically active. Typical procedures for activating the alloy are disclosed in the prior patent to Cambron and McKim No. 2,562,858.

Alternatively, the granular or particulate catalyst especially adapted for oxidation of ethylene by the fluidized bed technique may be prepared by using relatively porous non-silver particles, such as alumina, BeO, SiO carbon etc., granular alumina, the particles being largely hollow or otherwise formed so as to be highly porous. For example, alumina particles are first provided with a deposit of silver which may be provided by a chemical prealkaline earth metals and thereafter alloyed, the same 7 1y cleaned calcium metal.

as with granular particles of metallic silver described above. A preferable range of silver percentage of the supported catalyst granular mass would be about 2% to 20% with the range of calcium to silver being 5 to 75% and more preferably 7 to 15%.

These particles may then be treated with steam, hot water or acid to activate the catalytic alloy.

Such an impregnated granular catalyst is advantageous in a fluidized bed, as the catalyst is not appreciably abraded away due to the fact that the major portion of the alloy is protected by the irregular exterior outer surface of the hard alumina.

Three specific working examples are given hereinbelow to further describe the invention.

Example I A sheet of fine silver 0.040" thick and cleaned by acid pickling was placed in a space which also contained fresh- The whole system was heated to 640 C. when about of the calcium distilled was taken up by the silver sheet. The sheet was given no further annealing than the slow cooling in the furnace or about /2 hour at 625-640 C. The sheet was completely coated with calcium silver alloy with a core of pure silver remaining. The alloyed surface layer was broken up by rolling, the oxide removed by abrasion and the calcium removed by heating 2 hours in water at 200 C., leaching with hot 20% acetic acid and finally washing with water and drying.

Example II Asheet of silver 0.040" thick was cleaned by pickling in acid and placed in a common type of metallizing or vacuum coating apparatus. Distilled calcium stick freshly cleaned was placed on the evaporating tray. While the silver sheet remained substantially at room temperature, a layer of calcium was evaporated to a total thickness of 0.003" to 0.008", the calcium being heated to 500-600 ,C. in a vacuum of less than 3 X mm. mercury. The

sheet was then annealed in argon atmosphere one hour at 460 C., the surface layer broken up by rolling between roughened surfaces, sand blasted to remove surface oxide layers and activated as follows. The sheet was heated at 200 C. in water in an autoclave for 2 hours, boiled in 20% acetic acid, followed by water washing and drying.

Example III 100 cc. porous alumina granules were treated with 200 ml. 0.5 N AgNO solution, heated to boiling for minutes and cooled to room temperature. The following were then added in order: 18 ml. 28% NH OH, 100 ml. 2.4 N NaOH, 3 ml. 28% NH OH and 36 ml. of sugar solution made up by dissolving 8 gm. cane sugar in 80 ml. H O adding 10 ml. ethanol, 0.35 ml. cone. HNO boiling for 5 minutes and cooling. After standing one hour 3 volumes of water were added and the supernatant liquid decanted from the alumina. The granules were then dried at about 100 C. The resulting percent silver by weight was 9.6.

The granules were then spread in a thin layer not greater than about 3 mm. deep or tumbled continuously in an atmosphere of calcium vapor at a pressure of about 10-25 microns mercury for about 10 minutes, at about 640 0., held at temperature for about 30 minutes and then cooled. They were then activated by the normal autoclaving and steaming procedure followed by acetic acid leaching and washing with water.

The invention in its broader aspects is not limited to the specific process and steps described but departures may be made therefrom, within the scope of the accompanying claims, without departing from the principles of the invention and without sacrificing its chief advantages.

I claim:

, 1. A process of forming a catalytic unit comprising a metal base having thereon a coating of a silver-alkaline earth metal alloy catalyst for the oxidation of ethylenic hydrocarbons comprising vacuum depositing an alkaline earth metal in the form of a vapor on a metal base having a silver surface, subjecting the alkaline earth metal coated on the silver and the silver to a suflicient alloying temperature causing the alkaline earth metal to alloy with the silver and removing a portion of the alkaline earth metal to activate said alloy as a catalyst by treating said alkaline earth metal with an aqueous etching agent which is non-reactive with silver.

2. A process as defined in claim 1 wherein the alkaline earth metal is calcium and the alloying temperature is in the range of 450 to 640 C.

3. A process as defined in claim 1 wherein the alkaline earth metal is magnesium and the alloying temperature is in the range of 500 C. to 745 C.

4. A process as defined in claim 1 wherein the metal base is a silver sheet.

5. A process as defined in claim 1 wherein the metal base is particles of silver.

6. A process as defined in claim 1 wherein the metal base is a non-silver sheet coated with silver.

7. A process as defined in claim 1 wherein the metal base is non-silver porous particles coated with silver.

8. A process as defined in claim 1 wherein the metal base is heated to the alloying temperature during the vacuum deposition of the alkaline earth metal on the base.

9. A process as defined in claim 1 wherein the alkaline earth metal is deposited on a cold metal base after which the alkaline earth metal coated metal base is heated to an alloying temperature in an inert atmosphere causing the alkaline earth metal to alloy with the silver.

10. A process as defined in claim 1 wherein the pressure at which the alkaline earth metal is deposited on the metal base is in the order of 500 microns or less.

References Cited in the file of this patent UNITED STATES PATENTS 1,986,197 Harshaw Jan. 1, 1935 2,192,418 Sommer Mar. 5, 1940 2,561,411 Pfann July 24, 1951 2,562,858 Cambron et al July 31, 1951 2,599,978 Davis et a1. June 10, 1952 

1. A PROCESS OF FORMING A CATALYTIC UNIT COMPRISING A METAL BASE HAVING THEREON A COATING OF A SILVER-ALKALINE EARTH METAL ALLOY CATALYST FOR THE OXIDATION OF ETHYLENIC HYDROCARBONS COMPRISING VACUUM DEPOSITING AN ALKALINE EARTH METAL IN THE FORM OF A VAPOR ON A METAL BASE HAVING A SILVER SURFACE, SUBJECTING THE ALKALINE EARTH METAL COATED ON THE SILVER AND THE SILVER TO A SUFFICIENT ALLOYING TEMPERATURE CAUSING THE ALKALINE EARTH METAL TO ALLOY WITH THE SILVER AND REMOVING A PORTION OF THE ALKALINE EARTH METAL TO ACTIVATE SAID ALLOY AS A CATALYST BY TREATING SAID ALKALINE EARTH METAL WITH AN AQUEOUS ETCHING AGENT WHICH IS NON-REACTIVE WITH SILVER. 